ZOOLOGY AND BOTANY, MICROSCOPY, ETC. 743 



slide exactly on the centre of the diaphragm aperture. Tobacco smoke 

 is then slowly blown into the chamber formed by the slide and 

 glass ring, the chamber being immediately closed with a cover-glass 

 (fig. 127). In direct sunlight and very oblique illumination the smoke 

 particles then appear as countless white spots on a dark ground and 

 exhibit a dancing or treml)ling motion similar to the Browniau move- 

 ment in fluid. The stronger the light source the more visible are the 



Fig. 127. 



particles. They can also be seen with arc-light or incandescent gas, 

 or even with diffused dayhght. Although the size of the dark spot 

 stands in a certain proportion to the objective and the distance of the 

 optic plane from the eye, it is possible to obtain the effect without it 

 by merely shading the lower half of the mirror with one's finger, or 

 by interposing a piece of l)lack paper. The appearance of the field 

 resembles that of a star-lit sky. If direct incident illumination be 

 used the object is screened off by a black shade and the sunlight 

 allowed to pass through a small slit 5 mm. broad, l)eing previously 

 concentrated on it by an illuminating lens. The activity of the mole- 

 cular movement is influenced by the temperature of the particles, but 

 their gradual subsidence, due to gravity, is clearly noticeal)le under 

 the Microscope. The author experimented with many other vapours, 

 e.g., phosphorus and ammonium chloride. Bodaszewsky, who has also 

 worked at this subject, estimates the diameters of the smoke particles 

 at approximately 0*0002-0 '0003 /a, and therefore on the limit of 

 microscopical perception. The author, however, considers that they 

 are in general much larger. 



Microscopy : by E. J. Spitta.* — As will be gathered from the brief 

 description of its contents which follows, this book deals in a very 

 complete manner with the Microscope from both the mechanical and 

 the optical point of view, a characteristic feature being the exhaustive 

 treatment of many interesting aspects of the subject which have 

 hitherto been passed over hghtly, if treated at all. 



An introductory chapter, in which the various forms of simple 

 lenses and their general properties are described, is followed by one 

 dealing with the simple Microscope, which is illustrated in all its usual 

 forms. 



Chapters III. and IV. are devoted to the Compound Microscope, all 

 the mechanical details being discussed in the former, whilst the latter 

 describes the optical construction, and is distinguished by the thorough 

 way in which the various " corrections " are dealt with. 



* Microscopy : The Construction, Theory, and Use of the Microscope. By 

 Edmund J. Spitta. London : John "Murray, 1907, pp. xx. and 468 (17 pis. and 

 215 figs, in text). 



